1. Field of the Invention
The present invention relates in general to wellhead casing hangers and, in particular, to a casing hanger lockdown slip ring that converts axial loads into radial loads.
2. Brief Description of Related Art
Seals are used between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member may be a tubing hanger that supports a string of tubing extending into the well for the flow of production fluid. The tubing hanger lands in an outer wellhead member, which may be a wellhead housing, a Christmas tree, or a tubing head. A seal or packoff seals between the tubing hanger and the outer wellhead member. Alternately, the inner wellhead member might be a casing hanger located in a wellhead housing and secured to a string of casing extending into the well. A seal or packoff seals between the casing hanger and the wellhead housing.
A variety of seals of this nature have been employed in the prior art. Prior art seals include elastomeric and partially metal and elastomeric rings. Prior art seal rings made entirely of metal for forming metal-to-metal seals are also employed. The seals may be set by a running tool, or they may be set in response to the weight of the string of casing or tubing. One type of prior art metal-to-metal seal has inner and outer walls separated by a conical slot. An energizing ring is pushed into the slot to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead members. The energizing ring is a solid wedge-shaped member. The deformation of the inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent.
Thermal growth between the casing or tubing and the wellhead may occur, particularly with wellheads located at the surface, rather than subsea. The well fluid flowing upward through the tubing heats the string of tubing, and to a lesser degree the surrounding casing. The temperature increase may cause the tubing hanger and/or casing hanger to move axially a slight amount relative to the outer wellhead member or each other. During the heat up transient, the tubing hanger and/or casing hanger can also move radially due to temperature differences between components and the different rates of thermal expansion from which the component materials are constructed. If the seal has been set as a result of a wedging action where an axial displacement of energizing rings induces a radial movement of the seal against its mating surfaces, then sealing forces may be reduced if there is movement in the axial direction due to pressure or thermal effects. A reduction in axial force on the energizing ring results in a reduction in the radial inward and outward forces on the inner and outer walls of the seal ring, which may cause the seal to leak. A loss of radial loading between the seal and its mating surfaces due to thermal transients may also cause the seal to leak.
Prior art apparatuses that attempt to overcome the problems caused by axial movement of the casing hanger or tubing hanger include lockdown seals. Lockdown seals require formation of a groove in the landing sub or wellhead during the manufacturing process. After the wellhead and landing sub are positioned within the wellbore, the lockdown seal is run to the location of the landing sub where a ring of the lockdown seal either expands or contracts into the groove formed into the wellhead or landing sub, respectively. Unfortunately, the groove often fills with debris prior to run-in of the lockdown seal. The debris prevents engagement of the ring and thus, provides no lockdown benefits of the lockdown seal result.
Lockdown seals require a significant increase in production costs. This is due in part to increased costs to modify the basic wellhead or landing sub to include the lock ring groove. In addition, the use of these devices necessitate use of specialized tools and other components to properly land and engage the lockdown seal. Furthermore, prior art lockdown seals require some clearance between the landing sub and the lockdown apparatus of the lockdown seal. This clearance allows the lockdown seal to land in the appropriate location relative to the wellhead and landing sub while also providing the necessary space for the lockdown portion of the seal to engage either the wellhead or the landing sub. The clearance also allows the landing sub to shift before the lockdown device properly engages and arrests movement of the landing sub. In such instances, the landing sub may shift axially and cause the seal to fail. Thus, there is a need for a lockdown seal that overcomes the problems in the prior art described above.